Tuesday, January 21, 2014

The relaxation of striped spheres… ( #AIP_JCP #justpublished )

When was the last time that you took a bunch of pool balls, suspended them in a thick oil, and watched how they assembled? Pool balls being what they are, they will simply stack on themselves, though there is some question as to how efficiently they do so. If you start to shake the container, thereby maintaining some average kinetic energy (that is, temperature), they probably started to jiggle. They probably won't rotate much. Even if they do, it won't matter much because they collide with each other in the same way no matter what. So now take the balls and paint them with some pattern of red and blue paint, and suppose that there is a difference in the forces between the spheres depending on which colored surfaces are near each other. Now when they collide with each other, they have preferred relative orientation. The emergence of structure (or patterns in the positions and orientation) of the pool balls should presumably be very sensitive to how you painted them.

This is precisely the problem that Matthew Hagy and I have been studying over the past couple of years. Our pool balls are actually colloidal particles of a couple hundred nanometers in diameter. The paint corresponds to the charges encoded on the surface of the colloids. Opposite charges attract. Initially we studied Janus particles that literally have two faces, one hemisphere is positively charged and the other negative. (If interested, you can also check out my earlier blog post on the dynamics of Janus particles.) In the work that was just published in the Journal of Chemical Physics, we now consider the case in which the spheres are coated in stripes of alternating charge. This generalizes the surface pattern of the Janus particles to three, four, five, six, and more stripes. The funny thing is that very little happens to the packing of the particles because that property is so strongly dominated by the shape of the particles. But their motion, and the timescales in which they relax from a given deformation is highly sensitive to the number of stripes and perhaps also to how they are striped. In a sense, this says that if you want to maintain their behavior, you can fatten them up a little but you can't change their stripes.

The title of the article is "Dynamical simulation of electrostatic striped colloidal particles," and the work was funded by the NSF. It was released recently at J. Chem. Phys. 140, 034701 (2014), and featured on the cover!  Click on http://dx.doi.org/10.1063/1.4859855 to access the article.

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